1. Industrial Useful Field
This invention relates to a control system for an electric power supply which approximates an alternate input current to a sine wave by using a choke coil.
2. Prior Art
Generally, in an electric power supply which directly rectifies an alternate current to obtain a direct current, it has been required to eliminate a higher harmonic failure by improving a power factor on an alternate current receiving side because an alternate input current has had a pulse-shape including no idle period so that a power factor has not become 100%.
For this reason, an electric power supply having power factor improving function in itself has conventionally been proposed as illustrated by FIG. 3. This electric power supply has been controlled by a not-shown control circuit as diagrammed by FIG. 4. In FIG. 4; (a) represents a relation between a current 10i flowing into a choke coil L01 and an ideal sine wave 10a, (b) represents an operation timing of a semiconductor switch SW01, (c) represents an operation timing of a semiconductor switch SW02, (d) represents an operation timing of a semiconductor switch SW03, (e) represents an operation timing of a semiconductor switch SW04, and (f) represents a secondary output of a transformer Tr01, respectively. A voltage of an alternate current power supply 31 is supplied to the choke coil L01 through a rectifying circuit 32 composed of four diodes D01, D02, D03 and D04. Incidentally, the alternate input current is one in which high frequency components are cut from the current 10i flowing into the choke coil L01 by a capacitor C02. When the semiconductor switch SW01 is turned on and the semiconductor switch SW03 is turned off with the semiconductor switch SW02 turned on and the semiconductor switch SW04 turned off at a time t00, the current I0i flows through the semiconductor switches SW01 and SW02 to the choke coil L01 and an energy is accumulated therein so as to provide a mode gradually increasing the current I0i i.e. a boosting mode. When the semiconductor switch SW02 is turned off and the semiconductor switch SW04 is turned on with the semiconductor switch SW01 turned on and the semiconductor switch SW03 turned off at a time t01, the energy accumulated in the choke coil L01 circulates through the semiconductor switch SW01, a primary winding of the transformer Tr01 and the semiconductor switch SW04, and at the same time, is rectified by a diode D05 and transferred to a capacitor C03; so as to provide a mode gradually decreasing the current I0i i.e. a transfer mode. Then, when the semiconductor switch SW01 is turned off and the semiconductor switch SW03 is turned on with the semiconductor switch SW02 turned off and the semiconductor switch SW04 turned on at a time t02, the current I0i flows through the semiconductor switches SW03 and SW04 to the choke coil L01 and the energy is accumulated therein; so that the transfer mode shifts to the boosting mode again. Further, when the semiconductor switch SW02 is turned on and the semiconductor switch SW04 is turned off with the semiconductor switch SW01 turned off and the semiconductor switch SW03 turned on at a time t03, the energy accumulated in the choke coil L01 circulates through the semiconductor switch SW03, the primary winding of the transformer Tr01 and the semiconductor switch SW02, and at the same time, is rectified by a diode D06 and transferred to the capacitor C03; so that the boosting mode shifts to the transfer mode again, which decreases the current 10i gradually. When the same operations are repeated frequently, a secondary output of the transformer Tr01 is obtained as diagrammed by (f) of FIG. 4. In order to obtain the alternate input current such as the ideal sine wave diagrammed by (a) of FIG. 4, it may be enough to change a duty ratio of the boosting mode with the transfer mode and to cut high frequency components by the capacitor C02. Namely, it may be enough to increase an operation time of the boosting mode when increasing the alternate input current, and to increase an operation time of the transfer mode when decreasing the alternate input current. A conventional electric power supply having such a structure is disclosed in Japanese Unexamined Patent Publication NO. 63-190557.
In the foregoing conventional control system, a sum of the operation time for one boosting mode and that for the succeeding transfer mode is always constant, and the duty ratio of the transfer mode with the boosting mode is changed. In such a control system, however, it is necessary to reduce the duty ratio of the transfer mode with the boosting mode or that of the boosting mode with the transfer mode because a rate of change of the ideal sine wave I0a is maximum in the vicinity of a zero-cross of alternate current input voltage. Further, a waveform distortion during a period D shown by (a) of FIG. 4 is produced due to an unbalance between the foregoing energy and an energy transferred to the capacitor during the succeeding transfer mode or due to accumulation times of the semiconductor switches SW01, SW02, SW03 and SW04, because the energy accumulated in the choke coil L01 is small in the vicinity of the zero-cross of alternate current input voltage during the boosting mode.